As a hydraulic pump which is one example of the fluid pressure pump, a swash-plate type hydraulic pump has been known. The swash-plate type hydraulic pump includes a swash plate disposed at an oblique angle with respect to a shaft serving as a rotational shaft; a plurality of pistons that rotate about the shaft as they are in contact with the swash plate; and a cylinder block that houses the plurality of pistons and forms a cylinder chamber together with the plurality of pistons. The hydraulic pump may include a port plate that has an oil passage configured to be connected with an oil passage situated external to the hydraulic pump. The port plate is in contact with the cylinder block and may communicate a port(s) and the cylinder chamber.
In this type hydraulic pump, the shaft and the cylinder block are rotated together by an electric motor. As they rotate, the pistons slide on the swash plate in the rotational direction of the cylinder block and reciprocate in the axial direction and thus pumping. In this manner, inlet of the hydraulic fluid into the cylinder chamber and exhaust of the hydraulic fluid from the cylinder chamber are performed.
If the oil leaks between the cylinder block and the port plate, the pump efficiency of the hydraulic pump is decreased. Furthermore, if a plurality of pistons move away from the swash plate, it would take a long time for the plurality of pistons touch the swash plate again by the rotation of the cylinder block, which also decrease the pump efficiency.
To address this problem, Japanese Patent Application Publication 2013-177859 (hereunder referred to as “'859 Publication”) discloses a swash-plate type hydraulic pump that includes a pressing means for pressing the cylinder block to the port plate and a plurality of pistons to the swash plate. The pressing means includes a spherical movable member, a coupling ring through which the plurality of pistons are inserted, and a coil spring disposed between the movable member and the cylinder block. With the pressing means, the coil spring presses the movable member and the spherical surface of the movable member curved-contacts the coupling ring, which allows heads of the plurality of pitons to uniformly contact the swash plate as smoothly following the angle change of the swash plate. Moreover, the pressing means presses the cylinder block to the port plate. Therefore it is possible to prevent the oil from leaking between the cylinder block and the port plate.
In this hydraulic pump, the leakage of the oil between the cylinder block and the port plate (also referred to as a “valve plate”) should be prevented during two operational states: one is a start action state and the other is rotation state. During the rotation state of the hydraulic pump, the plurality of pistons reciprocate and the hydraulic pressure in the cylinder chamber is increased so that the hydraulic pressure in the cylinder chamber pushes the cylinder block toward the port plate. Consequently the coil spring, which is the pressing member, does not have to push the cylinder block toward the port plate and it only has to do is to maintain the state where the plurality of pistons are pressed to the swash plate. Therefore the coil spring does not have to exert a large spring force. Whereas during the start action state of the hydraulic pump, the hydraulic pressure in the cylinder chamber is low so that the hydraulic pressure in the cylinder chamber does not push the cylinder block toward the port plate. Accordingly the coil spring, which is the pressing member, has to push the cylinder block toward the port plate and has to maintain the state where the plurality of pistons are pressed to the swash plate. Therefore the coil spring has to exert a large spring force.
As described above, a different magnitude of spring force may be required during the different operational states such as the start action state and the rotation state of the hydraulic pump. More specifically, if a small spring force is applied by the coil spring during the start action state of the hydraulic pump, the cylinder block is not pushed toward the port plate with an appropriate force. In this case, the hydraulic fluid may leak between the cylinder block and the port plate. Whereas if a large spring force is applied by the coil spring during the rotation state of the hydraulic pump, the sum of the hydraulic pressure in the cylinder chamber and the spring force is applied to the cylinder block and consequently an excessive pressing force is applied to the cylinder block that pushes the cylinder block toward the port plate. This may cause the cylinder block to contact the port plate directly while the cylinder block rotates relative to the port plate, which may abrade the cylinder block and the port plate. In the hydraulic pump disclosed in the '859 Publication, a coil spring that applies a spring force required at the time of the start action of the hydraulic pump is used so that the above-mentioned problem may occur during the rotation of the hydraulic pump, which needs to be addressed. This problem is not limited to the hydraulic pressure but also applies to a fluid pressure for water and an air pressure for air. In other words, this problem relates to fluid used in the pump.